CAIE A Level Biology 9700 · Cell membranes and transport
A student investigates the uptake of nitrate ions by root cells of a plant. Root tissue is placed in two different concentrations of nitrate solution. At an external concentration of 0.2 mmol dm-3, uptake is rapid and is completely stopped when cyanide is added. At an external concentration of 20 mmol dm-3, uptake is also rapid but is unaffected by the addition of cyanide. Explain these observations in terms of the different mechanisms of transport involved.
6 marks, so the mark scheme is looking for about 6 separate creditworthy points. Making the same point twice in different words earns it once.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
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